A Method for Calculating Frontal A and B Stiffness Coefficients from NCAP Barrier Crash Tests Accounting for Bumper Cover Air Gap 2024-01-2473

A and B stiffness coefficients to model the frontal stiffness of vehicles is a commonly used and accepted technique within the field of collision reconstruction. Methods for calculating stiffness coefficients rely upon examining the residual crush of a vehicle involved in a crash test. When vehicles are involved in a collision, portions of the crushed vehicle structures rebound from their maximum dynamic crush position. Once the vehicle structures have finished rebounding, the remaining damage is called the residual crush. A problem can arise when the plastic bumper cover rebounds more than the vehicle's structural components, resulting in an air gap between the structural components and the plastic bumper cover. Most modern New Car Assessment Program (NCAP) tests quantify crush in the test reports based on the deformed location of the plastic bumper cover and not the structural components behind the plastic bumper cover. This results in an underreporting of the actual residual crush. The stiffness coefficients developed from these under-reported crush measurements over-report the actual stiffness of the vehicle. Features of the NCAP test procedure, including the incorporation of load cells on the face of the non-deformable impact barrier, accelerometers located in the test vehicle, and the software provided by the National Highway Traffic Safety Administration (NHTSA) to analyze the impact data, allow the analyst to quantify the true residual crush sustained by test vehicles. This paper presents a new methodology for developing frontal A and B stiffness coefficients based on NCAP test data. In the evaluated NCAP tests where the NHTSA took residual crush measurements to the front bumper covers of the test vehicles, the average residual crush from the proposed method was 3.6 ± 3.2 inches greater than that of NHTSA-reported measurements. In seven additional NHTSA frontal fixed barrier tests in which residual crush measurements are taken directly to the front bumper beam, the average residual crush from the proposed method is 0.2 ± 1.4 inches greater than that of NHTSA-reported measurements. A and B stiffness coefficients were calculated using this method for 14 NCAP tests and are presented.